Old advice is new again.

Banks are going to have look beyond the loan if they want to keep milking corporate lending for profits, according to a new study.

Pleas to cross-sell have been heard before, but business lending had been so strong in the postcrisis era relative to everything else that the extra effort seemed less imperative. Yet the longtime bright spot has lost some luster lately.

Returns for commercial loans have gradually tightened, to 2.7% last year from 3.2% two years earlier, according to a study released Monday by the Boston Consulting Group.

Loan growth has been robust, but not revenues. Loan volume increased by an average of 10% each year from 2011 to 2014, but revenue grew by just 3% on average, the study said.

Corporate banking still offers healthy returns, "but in the last couple years it has gotten a little less profitable, mostly due to margins," said Pieter van den Berg of the Boston Consulting Group, one of the authors of the report.

Because lending is becoming less profitable, providing other revenue-generating services — like treasury management and transaction banking — is more important than ever.

"Lending is still the anchor product, but the money you make on that credit, especially on a return-on-capital basis, has gone down," van den Berg said. "The cross-sell has gone from being a bonus to a prerequisite for having a profitable relationship."

Corporate profits have grown faster than consumer spending since the recession, so banks have turned their focus to commercial clients, and that is unlikely to change even if profit margins keep shrinking. Corporate banking is still a massive revenue driver, accounting for about half the revenue of the banking industry globally, the report says.

However, it is less profitable than it was a few years ago, mostly because of margin compression and higher regulatory costs, van den Berg said. Three-quarters of North American corporate banking divisions have seen their profit shrink over the last three years, according to the consulting firm's benchmarking analysis, which draws from public and proprietary information.

The gap between winners and losers is widening globally. Among corporate banks in all regions, the outperformers in return on assets increased their lead on the laggards from 2007 to 2013.

Differences in ROA have been steadier in North America, but even in the U.S., there is a wide gap between the top-performing and the struggling corporate-banking divisions, van den Berg said.

The top performers have been able to do more than cross-sell more products to their customers. Increasingly, they tend to be the banks that do not try to be everything for every client and have homed in on a few industries and products that they can excel in, van den Berg said.

"We've seen that the winning players have been bringing much more industry-specific knowledge and solutions," he said.

Successful banks have also been offering this industry-specific expertise in hot sectors, like health care lending, to smaller clients, he said. Formerly, industry-specific bankers focused on the largest clients, but "we've seen it trickle down into the middle-market space," van den Berg said.

Two keys to staying in the winner's circle over the next several years will be managing regulatory expense as efficiently as possible and keeping digital offerings up to date, the report said.

Digital — from product design to use of big data — is where a lot of banks can separate themselves from the pack. Seventy percent of commercial clients said a bank's digital capabilities are important to them, according to the study. Van den Berg has seen more banks offering tailored digital services — websites or mobile apps — to individual clients, which was once rare.

Van den Berg also argues that more analytical pricing of commercial loans could be a margin booster. Subjective factors, as opposed to quantities risk analysis, play a large role in many banks' loan pricing, the study found. The result can be a skewed pricing structure.

"We've seen big discrepancies about what's charged to one client or another that are not necessarily driven by the value of the relationship," van den Berg said.

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Corrected March 24, 2015 at 10:46AM: A description of the share of banks' revenue that comes from corporate banking was corrected from an earlier version. So was the description of the niche markets that Pieter van den Berg said have been targeted by successful corporate bankers.